1. Field of the Invention
The present invention relates to an image forming apparatus having a post-processing device for aligning a plurality of sheets of recording material and performing predetermined post-processing.
2. Description of the Related Art
In conventional image forming apparatuses, some apparatuses have a post-processing device for aligning a plurality of sheets of recording material on which image formation is performed, performing post-processing such as stapling, and punching, and for discharging them. In such post-processing devices, the image formation is performed in a main body of the image forming apparatus, and the plurality of sheets of discharged recording material are received and stacked. The stacked recording materials are aligned by an alignment member. On the aligned recording materials, post-processing such as stapling and punching is performed, and the recording materials are discharged.
For such post-processing devices, Japanese Patent Application Laid-Open No. 2004-85982 has proposed a method for controlling an alignment position (movement distance of an alignment member) or an alignment speed (movement speed of the alignment member) depending on the type of the paper (thickness and surface properties) in order to prevent alignment failure or stacking failure due to a difference between friction force on surfaces of the recording materials.
In Japanese Patent Application Laid-Open No. 2004-85982, when stacked recording materials are aligned by the post-processing device, the alignment position (movement distance of the alignment member) or the alignment speed (movement speed of the alignment member) is controlled depending on the type of the paper, and each time a sheet of recording material is stacked, the alignment processing is performed once or several times.
However, depending on the type of the recording material or environmental conditions, if the alignment processing is performed each time a sheet of recording material is stacked, static electricity is produced due to friction between the recording material and the alignment member. As a result, the alignment performance is reduced. FIG. 8 illustrates an example of the alignment member in the post-processing device. The alignment member illustrated in FIG. 8 aligns the recording materials by moving in the arrow directions in the drawing. When the recording materials are aligned by such an alignment member, due to the friction between the alignment member and the recording material, the static electricity is produced. Then, static clinging may occur between the alignment member and the recording material. Especially, if the contact area between the alignment member and the recording material increases, a greater amount of the static electricity is produced. Accordingly, it is highly possible that the static clinging between the alignment member and the recording material occurs. If the static clinging between the alignment member and the recording material occurs, as illustrated in FIG. 9, the recording materials are not correctly aligned, in other words, alignment failure occurs.